The plasma cutting method has been popular in which plasma arc is ejected toward a material to be cut in order to cut it. The plasma cutting apparatus which is used when the plasma cutting method is performed has a conductive electrode which is detachably attachable to a conductive electrode base provided on a torch main body of the plasma cutting torch, a conductive nozzle which is detachably attachable to the torch main body, the nozzle surrounding the electrode and being disposed on the electrode in an insulated state, and the gas supplying source which supplies plasma gas for forming plasma arc in the circumference of the electrode.
When a material to be cut is cut by the plasma cutting torch, a pilot arc is firstly formed by producing an electric discharge between the electrode and the nozzle while providing plasma gas between them. A pilot arc is ejected toward the material to be cut from the nozzle and an electric discharge occurs between the electrode and the material to be cut, thereby forming a plasma arc. By melting the base material with the plasma arc and by eliminating molten material while moving the plasma cutting torch, the material to be cut can be cut.
In the plasma cutting torch in which plasma gas including oxygen is ejected in a plasma state toward the material to be cut to cut it, an electrode is commonly used in which an electrode member formed of hafnium, zirconium, or alloy thereof is buried and fixed in the center of a holder formed of copper or copper alloy. An electrode member of such an electrode is worn as it is melted by the heat of the plasma arc and oxidized by the plasma gas. Thus, an extension of service life of the electrode of the plasma cutting torch is desired and many proposals are made for it.
The patent document 1 discloses the plasma cutting starting method which is accomplished based on the knowledge that an endurance time becomes shorter as the number of start operations becomes large. In this technology, when starting the plasma cutting, a gas in which oxygen gas with 70 to 10 mole percent and nitrogen gas with 30 to 90 mole percent are mixed is used and after the completion of the start, the gas is switched to oxygen gas (plasma gas) with the concentration of 95 percent or more to perform the cutting.
The patent document 1 discloses the gas supply piping system (FIG. 2) and the following explanation. When starting the plasma cutting, the mixed gas electromagnetic valve 9 is opened and the oxygen electromagnetic valve 8 is closed. After the completion of the start of cutting, the mixed gas electromagnetic valve 9 is closed and the oxygen electromagnetic valve 8 is opened thereby switching plasma gas to oxygen. The switching is performed by a setting timer according to previously prepared widths of the plates because the piercing time depends on the width of the plate to be cut.
The patent document 2 relates to a method and an apparatus for cutting and welding with plasma arc. The patent document 2 discloses that the nitrogen or oxygen is supplied to the oxygen plasma supply line (76) by the nitrogen/oxygen solenoid selector valve SV15, that the solenoid selector valves SV1 to SV3 are connected to the supply line in parallel and that the needle valves MV1 to MV3 for adjusting a flow rate are respectively connected to the solenoid selector valves SV1 to SV3 in series.
When a start command of the operations is issued, a pilot arc is started by opening SV2 and SV3 at the same time and by supplying a gas selected by SV15 and whose flowing amount is previously set by the needle valves MV2 and MV3 to the circumference of the electrode. Thereafter, SV3 is closed when a predetermined time elapses, and SV1 and SV3 are opened in addition to SV2 when a predetermined time further elapses, and then SV2 is closed, and SV3 is closed when the pilot arc is switched to the plasma arc thereby continuing the plasma arc by the gas supplied from SV1.